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Biophys. J. BioFAST: First Published December 13, 2004. doi:10.1529/biophysj.104.050047
© 2004 by the Biophysical Society.

A more recent version of this article appeared on March 1, 2005.
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Michael A. Ferenczi
Theyencheri Narayanan
Manfred Roessle
Sergey Y. Bershitsky
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MUSCLE AND CONTRACTILITY

Strong Binding of Myosin Heads Stretches and Twists the Actin Helix

Andrey K. Tsaturyan 1, Natalia Koubassova 2, Michael A. Ferenczi 3, Theyencheri Narayanan 4, Manfred Roessle 4 and Sergey Y. Bershitsky 5*

1 Institute of Mechanics, Moscow University
2 Institute of Mechanics, M.V. Lomonosov Moscow State University, Russia
3 Imperial College London, UK
4 ESRF, France
5 Institute of Immunology and Physiology UB RAS

* To whom correspondence should be addressed. E-mail: syb{at}efif.uran.ru.

Submitted on July 22, 2004
Revised on September 9, 2004
Accepted on 29 November 2004


  Abstract
Calculation of the size of the power stroke of the myosin motor in contracting muscle requires knowledge of the compliance of the myofilaments. Current estimates of actin compliance vary significantly introducing uncertainty in the mechanical parameters of the motor. Using X-ray diffraction on small bundles of permeabilized fibers from rabbit muscle we show that strong binding of myosin heads changes directly the actin helix. The spacing of the 2.73-nm meridional X-ray reflection increased by 0.22% when relaxed fibers were put into low-tension rigor (<10 kN/m2) demonstrating that strongly bound myosin heads elongate the actin filaments even in the absence of external tension. The pitch of the 5.9-nm actin layer line increased by ~0.62% and that of the 5.1-nm layer line decreased by ~0.26% suggesting that the elongation is accompanied by a decrease in its helical angle {vartheta} (~166°) by ~0.8°. This effect explains the difference between actin compliance revealed from mechanical experiments with single fibers (Linari et al., 1998) and from X-ray diffraction on whole muscles (Takezawa et al., 1998; Bordas et al., 1999). Our measurement of actin compliance obtained by applying tension to fibers in rigor is consistent with the results of mechanical measurements (Linari et al., 1998).

Key Words: actin filament compliance, muscle fiber, muscle mechanics, x-ray diffraction




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Copyright © 2004 by the Biophysical Society.